Erythropoietin is a growth factor that has revolutionized the management of anemia in patients with end-stage renal disease (ESRD). A significant clinical challenge that remains in some patients is the relative resistance to erythropoietin, which leads to use of successively higher erythropoietin doses and increased risk of adverse outcomes. Chronic inflammation is an important factor contributing to erythropoietin resistance, yet the molecular pathways mediating this phenotype are unclear. Erythropoietin acts through the erythropoietin receptor (EpoR) present in erythroblasts. Importantly, alternative mRNA splicing produces a soluble form of EpoR (sEpoR) that is present in human blood. While the function of sEpoR is unknown, sEpoR may modulate erythropoietin signaling, raising the possibility of a physiologic role for this soluble receptor. No studies have systematically examined sEpoR levels in ESRD. Using archived serum samples obtained from subjects with ESRD, we have generated preliminary data to show that sEpoR is detectable as a 27kDa protein in their serum, and that higher serum sEpoR levels correlate with increased erythropoietin requirements. In addition we have preliminary data suggesting that sEpoR inhibits erythropoietin mediated signal transducer and activator of transcription 5 (Stat-5) phosphorylation in cell lines expressing EpoR. We also demonstrate that serum from patients with elevated sEpoR levels blocks this phosphorylation in ex vivo studies. The intent of this proposal is to confirm that serum with high levels of sEpoR can block erythropoietin mediated intracellular signaling in vitro by rescue with exogenous erythropoietin and inhibition of the effect after immunoadsorption of sEpoR from the serum. We will also examine the regulation of sEpoR secretion in response to inflammatory mediators known to be elevated in ESRD. Finally, we will perform two clinical studies using archived samples from large dialysis cohorts (ArMORR, US; 4D, Germany) to test the hypothesis that elevated sEpoR levels at the start of dialysis independently predict subsequent erythropoietin dose. We believe this exploratory R21 mechanism will permit a collaborative team of basic scientists and clinical investigators to address one of the most common and vexing problems faced by ESRD patients. This proposal has the potential to lead to changes in the diagnosis and management of patients with erythropoietin resistance.